The present invention relates to a closed battery which is capable of bursting at a predetermined pressure, particularly a low pressure, so as to release rapidly the internal pressure in the battery and at the same time to interrupt the current when the battery temperature or the battery internal pressure is elevated.
Recently, with increasing requirements for electronic devices that are small and light in weight with high performance, non-aqueous batteries with high energy density have become popular. Batteries which use an alkali methal such as lithium, sodium, or potassium as an active material for the negative electrode thereof are used in a battery container with a completely closed structure, since the alkali metal readily reacts with water.
These completely closed batteries have the advantage or reasonable storability. However, the fact that they are well sealed sometimes causes the battery to rupture because of abnormally elevated internal temperatures; when a short circuit is formed inside or outside of the battery, causing a large flow of current; or hydrogen is generated in the battery because of water contamination during the manufacturing process thereof. In this case, the device that accommodates a rupturing battery will be damaged and, moreover, there may be a danger of the broken device and/or the battery harming the user. Accordingly, there is a need for a pressure releasing mechanism before the internal pressure of the battery is increased to such a degree that the battery ruptures.
So far, several methods for releasing the internal pressure of a battery have been proposed, and some of them have been used in practice. For instance, Japanese laid-open publication HEI 2-304861 discloses a safety valve for a battery which comprises a valve chest provided a part of a container of the battery and having a valve through hole which communicates with the inside of the battery and an outlet which communicates with the outside of the battery. The valve chest comprises a valve member, the surface of which opposed to the valve through hole is made of rubber, and an elastic member for pushing the above mentioned rubber surface of the valve member to the valve through hole. This safety valve maintains the sealability of the battery by closing the valve through hole with the rubber valve member, and prevents the internal pressure of the battery from rising in excess of a predetermined pressure by permitting the valve through hole to be opened when the internal pressure rises to the pressure prescribed for the elastic member.
Such a conventional safety valve has been widely used in Nixe2x80x94Cd batteries. However, this type of safety valve has not yet been used in non-aqueous batteries which use alkali metals as the active material for the negative electrode, since, even if the elastic member presses the valve member to the valve through hole to close the battery, it is impossible to obtain the high sealability required for non-aqueous batteries.
One method for releasing the internal pressure of a battery which requires high sealability is disclosed in Japanese laid-open publication SHO 63-285859. In this method, a container of the battery is provided with a thinned portion in part of the wall thereof This thinned portion is formed by cold rolling a sheet of the container using a press device until the thickness of the rolled portion of the sheet is half the thickness of the initial thickness of the sheet. Thus, the battery is provided with good sealability with regard to this thinned wall portion because it is only transformed from a part of the continuous wall of the container.
Another pressure-releasing mechanism of this type is disclosed in Japanese laid-open publication HEI-6-215760. This releasing mechanism comprises a valve diaphragm fitted in an open end of the cylindrical container of a battery having a bottom and disposed above the electrode element of the battery, along with a lead for current cutoff disposed above the valve diaphragm. In this structure, when the internal pressure of the battery is elevated, the valve diaphragm expands to break the lead for current cutoff to interrupt the current flow.
According to the method disclosed in Japanese laid-open publication SHO-63-285859, it is necessary to obtain an extremely thin wall portion of the container so that the thinned portion can be ruptured at a relatively low pressure. However, when the wall portion is made too thin, fine or minute cracks may form during press-forming, thus impairing the sealability of the container.
Further, when metals are processed by cold working, their workability will be inevitably harder. Metal hardening does not always take place uniformly, and therefore, there arises a problem in that the operating pressure for such pressure releasing mechanisms cannot be controlled to the degree desirable to avoid damage. While half-etching has been proposed for thinning part of the wall of a battery container, it is extremely difficult to control the remaining thickness of the thinned portion after etching, and it is also difficult to obtain a satisfactory yield. Another problem is that the half-etched portion is not necessarily free from pinholes, and therefore, all of these products must be carefully inspected prior to use.
On the other hand, according to the method disclosed in Japanese laid-open publication HEI 6-215760, it is necessary to accurately control the depth of a cut off portion formed in the lead, which makes it difficult to form the cut off and interruption of the current is not reliable at a predetermined pressure.
In order to solve the above-mentioned problem, the present invention provides a closed battery which comprises an electrode element consisting of a positive electrode, a negative electrode, and a separator; an electrolyte; a battery container accommodating the electrode element together with the electrolyte; and a closing member fitted in the inner periphery of an open end portion of the battery container to close the open end portion of the battery container. The closing member consists of a metal substrate, a valve element provided in the metal substrate and defined by a break line so as to serve as a releasing chip such that when the internal pressure of the battery is elevated, the valve element is bent from a bending fulcrum so as to provide the metal substrate with an opening portion for releasing the internal pressure, and a metal foil which is adhered to the inner surface of the metal substrate.
According to the present invention, when the internal pressure of the battery is elevated because of a short circuit, overcharge, reverse charge, or the like, a valve chip, consisting of a metal substrate in which a valve element is provided for releasing the internal pressure of the battery, and a metal foil which has a uniform and accurate thickness, and is laid over the metal substrate so as to close a through hole which is usually closed with the valve element, operate to deform itself and push and raise the valve element up. When the internal pressure reaches a predetermined pressure, the metal foil stably and accurately bursts to cut the connection and at the same time discharge the internal gas within the battery, thereby preventing an abrupt rising of the internal temperature and possibly exploding the battery.